I've been having no luck in finding out what the most popular handguns and semi auto rifles in the US. I want to know this as this will dictate what the most common ammunition remaining is.

I've read some articles which are confusing, but it's pretty obvious the AR-10/15 is heads up letting me confidently assume* that 5.56 x 45mm is the most popular and this seems to be followed by the venerable 7.62 x 51mm is at least equal. After this I just don't know enough about the US to guess.

Shotguns are comfortably 12 gauge with other far behind.

Pistols are where it gets murky. Some years ago I would have confidently stated that 9.0 x19mm was everywhere but since then .40 cal round has been making headway. Couldn't even guess after that.

As far as pistols are concerned I think the 9x19mm is most popular for military, conceal carry users, and police forces (local, state, federal). A significant number of police forces (local) and conceal carry users use the .40 S&W.

Rifles will be .223/5.56, 7.62X39, .30-30, .308/7.62X51, .30-06 and .303 for the Canadian side.
Pistols will be 9X19, .38 Special, .357 Mag, .45 ACP. these are mostly age and sheer amount. M1911's, Hp35's, are prolific, Glocks and "N" frame Smith and Wessons will last forever.
Black powder replicas, Remington, Colts(need to stat), the rifles. For replica revolvers, the barrels are .451, but the chambers are .455, so .451 balls creep forward and "jam" the cylinder. (worthless firearms trivia time). The .31 cap and ball revolver could replace the .22LR in small game, pest removal.

Keep in mind that's 5th by revenue, not by number of rounds. Right now 9mm is around 15 to 18 cents per round at Lucky Gunner ... .22 LR is about 5 cents per round. By round count, .22 LR looks to be about as popular as 9mm.

I think the High Standard 10B bullpup shotgun may be replaced by this Mossberg 500 bullpup conversion;

Either of these are unlikely to replace the High Standard 10B. The High Standard 10B is a gas operated semi-automatic that can be fired one-handed. Both of these are pump action. In cases where you may only have one hand free, carrying an unconscious person or fragile lab samples, that could be valuable.